Engineers and scientists are frequently charged with the task of metering liquid flows in closed conduits that surcharge. For example, sanitary sewers that carry wastewater from areas that experience severe infiltration/inflow during wet weather typically operate under surcharged conditions during rainfall events. Similarly, culverts under roadways and storm sewers often operate as partially full open channels during dry weather but operate as full pipes during wet weather.
Applicant has previously disclosed a novel meter capable of metering flows in pipes that surcharge in U.S. Pat. Nos. 4,799,388 and 4,896,542. The meter is also described in a report entitled: "The Flumeter.TM.: A New Tool for Wastewater Management" prepared for the U.S. Department of Energy Energy-Related Inventions Program by Yellowstone Environmental Science, Inc., Bozeman, Montana, May 1988.
The inventions disclosed in the above referenced documents rely on three conventional bubbler systems to transmit pressures produced by the primary element, a combination Venturi flume/Venturi tube of novel design, to two differential pressure sensing means in the secondary element or instrument. Alternatively, three piezoelectric pressure transducers may be used. With this option, electrical signals characterizing the pressures are transmitted to the secondary element.
Subsequent research has revealed that the disclosed pressure sensing methods have certain limitations. The three bubbler system has a higher power requirement than is desired for a portable flow meter. Furthermore, rapid increases in flow rates can cause momentary inaccuracies in pressure readings. The use of three submerged piezoelectric pressure transducers can also introduce inaccuracies because of the large range over which they must operate.